The invention provides a sheet of material designed to be bent along bend lines. The sheet of material is formed with a plurality of slits which generally designated along and approximated to a bend line. Each slit has a slit end which gradually extends in a direction away from the bend line so as to define a tongue portion. The tongue portion is laterally offset of a plane where the sheet of material is located before the sheet of material is bent. The invention enables bending of a sheet of material precisely, and the bent part has substantial load-bearing capacity and excellent fatigue-resistant performance.